In a power transmission system of automobiles and various industrial machineries, a constant velocity universal joint connects a rotating shaft on a driving side and a rotating shaft on a driven side, and transmits torque at a constant speed. As a ball-type constant velocity universal joint using a ball that serves as a torque transmitting element, various types, such as a Birfield-type constant velocity universal joint (BJ), a double-offset type constant velocity universal joint (DOJ), and a Löbro-type constant velocity universal joint (LJ), are known. Six or eight torque transmitting balls are commonly used.
These constant velocity universal joints include an outer joint member, an inner joint member, torque transmitting balls, and a cage as main constituent elements. Track grooves that extend in an axial direction are formed on an inner circumferential surface of the outer joint member. Track grooves that extend in the axial direction are formed on an outer circumferential surface of the inner joint member. The rotating shaft on the driving side and the rotating shaft on the driven side are connected to the outer joint member and the inner joint member. A track groove on the outer joint member and a track groove on the inner joint member form a pair, and form a ball track. A torque transmitting ball is set on each ball track. The torque transmitting balls are housed within pockets formed in the cage in the circumferential direction and are held such as to roll freely.
In recent years, size reduction and weight reduction of such constant velocity universal joints are being achieved. However, the most difficult aspect of achieving size reduction and weight reduction of the constant velocity universal joint is maintaining sufficient strength at high operating angles. High angle strength is evaluated through quasi-static torsion testing performed on the constant velocity universal joint (Patent Document 1: Japanese Patent Laid-open Publication No. 2000-104749). Here, quasi-static torsion testing refers to a test for measuring breaking torque, in which toque is applied while the constant velocity universal joint is being rotated, taking into consideration actual vehicular conditions.
Quasi-static torsional strength is heavily dependent on strength of carburized components, such as the cage and the inner joint member. Thus, toughening the carburized components (particularly the cage) becomes an issue when achieving size reduction and weight reduction of the constant velocity universal joint. Therefore, in general, to manufacture the cage of the constant velocity universal joint, after cold plastic working is applied to a low carbon steel material (SCr 415), machining is performed. Carburizing and quenching is then performed, and finishing is performed (Patent Document 2: Japanese Patent Laid-open Publication No. Heisei 5-331616).
As a measure for increasing strength of carburized components, for example, a technique described in Patent Document 3 (Japanese Patent Laid-open Publication No. 2002-371320) is proposed. In other words, when a component on which cold plastic working has been applied is held at a high temperature for a long period of time, austenite grain coarsening occurs and strength is significantly reduced. Therefore, a special element is added to the material, thereby suppressing growth of austenite grains and increasing strength.
A processing method for improving strength is proposed (Patent Document 4: Japanese Patent Laid-open Publication No. 2005-48292) in which, after carburization to 0.6% to 0.9% carbon concentration, high-frequency hardening is performed, thereby reducing segregation of carbide and the like, and miniaturizing austenite grains (#10 or more).
Moreover, a technique is also proposed (Patent Document 5: Japanese Patent Publication No. 3381738) in which mechanical strength is increased by high-frequency hardening being performed after nitriding treatment. A technique is also proposed (Patent Document 6: Japanese Patent Publication No. 3395252) in which annealing is performed after carburization, and high-frequency hardening is performed on only the surface, thereby increasing strength.